Philosophy and the Sciences
Introduction to the Philosophy of the Physical Sciences
Dark Matter and Dark Energy
What kind of entities are dark matter and dark energy?
What is the rationale for choosing between rival research programs?
The ‘concordance model’ or ΛCD
This asserts that we live in an infinite universe, with
approximately 5% ordinary matter (baryons), 25% Cold
Dark Matter, and 70% Dark Energy.
It builds on Einstein’s general relativity (GR) and the socalled Friedmann-Lemaître-Robertson-Walker (FLRW)
models.
Thomas Kuhn on the rationality of
theory-choice
In The Essential Tension (1977), Kuhn pointed out that
theory choice seems governed by the following criteria:
• accuracy (i.e. the theory is in agreement with experimental evidence);
• consistency (i.e., the theory is consistent with other
accepted scientific theories);
• broad scope (i.e., the theory has to go beyond the original phenomena it was designed to explain);
• simplicity (i.e., the theory should give a simple account
of the phenomena);
• fruitfulness (i.e., the theory should be able to predict
Novel phenomena).
Lambda-Cold Dark Matter by Coldcreation (CC-BY SA 3.0)
What are dark matter and dark energy?
• Evidence from Supernova Ia shows that not only is the
universe expanding but it is also accelerating in the expansion. The currently most accepted hypothesis for this
accelerated expansion is dark energy.
• Dark energy means giving weight to the vacuum. If
empty space can have density, it can also have anti-gravitational properties.
• Why do galaxy clusters, and superclusters exist? The
simplest answer is: because gravity can amplify density
irregularities.
• Kuhn: these criteria are either imprecise (e.g., how to
define ‘simplicity’?); or they conflict with one another
(e.g., while Copernicanism seems preferable to Ptolemaic
astronomy on the basis of accuracy; Ptolemaic astronomy
fares better on the score of consistency with the Aristotelian- Archimedean tradition of the time).
• These five joint criteria are not sufficient to determine
theory choice.
• Instead, external, sociological factors seem to play a
decisive role in how scientists gather consensus around a
given theory.
Going back to cosmology…
• By mapping the three dimensional distribution of galaxies, we’re able to infer the density of the universe.
What is the current evidence for the concordance model,
with dark matter and dark energy?
• The majority of the clumpable matter in the universe
is of a form that we only see through gravity. The matter
we see from radiation is dark matter (DM). Evidence for
Dark Matter comes from galaxies’ flat rotation curves.
Are there viable alternatives to the concordance model?
How do cosmologists make their rational decision of
endorsing the concordance model?
Alternatives to ΛCDM
Going back to Duhem and the underdetermination problem. Does underdetermination loom in contemporary
cosmology?
Two possible rivals to Dark Energy
1. Inhomogeneous Lemaitre-Tolman-Bondi (or LTB)
models (rather than the standard FLRW model, which
assumes, with the Cosmological Principle, that the Universe is roughly homogeneous and isotropic, namely it
has the same uniform structure in all spatial positions and
directions).
2. LTB denies homogeneity (but retains isotropy), and
assumes that there are spatial variations in the distribution of matter in the Universe, and that we live in an
underdense or ‘void’ region of the Universe (a ‘Hubble
bubble’), which is expanding at a faster rate than the
average.
• Instead of modifying FLRW models, we could try to
modify General Relativity itself (to avoid DE).
Is theory-choice underdetermined in
cosmology?
The underdetermination argument challenges the rationality of theory-choice.
Was Kuhn right in claiming that neither simplicity, nor
any of the other criteria, will ever be sufficient to determine the rationality of theory choice? Philosophers of
science have sometimes appealed to the notion of empirical support as a more promising way of thinking about
theory choice.
The concordance model is empirically supported not just
when there is direct experimental evidence for some of its
main theoretical assumptions, but when the model is integrated / embedded into a broader theoretical framework.
In this way, the model receives indirect empirical support
from any other piece of evidence, which although not a
direct consequence of the model itself, are nonetheless
consequences of the broader theoretical framework in
which the model is embedded.
Possible rivals to Dark Matter:
Modified Newtonian Dynamics or MOND, first proposed by Milgrom (1983), and in its relativistic form by
Bekenstein (2010).
Credit: Lahav, O., Massimi, M. “Dark Energy, Paradigm Shifts, and the Role of
Evidence” (2014)